The production of shaped articles having an impervious surface layer by the isocyanate-polyaddition process is known. It is carried out, for example, by introducing a reactive and, optionally, foamable mixture based on compounds containing several reactive hydrogen atoms and polyisocyanates into a mold (cf. for example, German Auslegeschrift No. 1,196,864 [British Pat. No. 969,114]). The compounds containing reactive hydrogen atoms used in the above reference are preferably polyethers and polyesters containing hydroxyl groups and the polyisocyanates used are, for example, 2,4- and 2,6-tolylene diisocyanate and isomer mixtures thereof. Also suitable are the polyphenyl-polymethylene-polyisocyanates obtained by condensing aniline with formaldehyde, followed by phosgenation. Water and/or fluorochlorinated hydrocarbons, for example, may be used as blowing agents. Catalysts known in the art for the production of polyurethanes are also generally used.
By suitably selecting the starting components, it is possible, by this process, to produce elastic products, rigid products and also variants falling between these two extremes.
As explained in German Offenlegungsschrift No. 2,622,951 (British Pat. No. 1,534,258, it is possible to use systems containing diamines as chain-extending agents, but generally not by a one-shot process. Where diamine chain-extenders are used, it is necessary first to prepare a "prepolymer" containing NCO groups. This "prepolymer" is then reacted with the diamine in a second stage to form the high molecular weight elastomer (German Auslegeschrift No. 1,240,654 [U.S. Pat. No. 3,428,610]).
The production of heavily stressed moldings generally requires the use of starting materials having a slightly branched structure which, after processing, give materials having a property spectrum resembling that of elastomers. Such moldings have been commercially produced for some time (for example, as soles in the shoe industry). Large moldings are used in the automobile industry.
The starting materials are processed into body work sections primarily by the so-called "reaction injection molding" (RIM) process. This process is a filling technique in which the highly active, liquid starting components are rapidly injected into the mold through high output, high pressure metering units after mixing in so-called "static impingement" mixing heads.
A detailed description of the reaction injection molding process may be found, for example, in the following publications: Piechota/Rohr: "Integralschaumstoffe (Integral Foams)", Carl Hanser Verlag, Munich/Vienna 1975; Prepelka/Wharton: "Reaction Injection Molding in the Automotive Industry", Journal of Cell. Plastics, Volume II, No. 2, 1975; Knipp: "Plastics for Automobile Safety Bumpers", Journal of Cell. Plastics, No. 2, 1973.
It is possible by the reaction injection molding (RIM) technique to produce large moldings weighing from 3 to 10 kg and more. These large moldings are used, for example, in the form of flexible body work sections in the automotive industry. They are known in the auto industry as so-called "soft face" elements, i.e. reversibly formable front or rear parts of motor vehicles.
Rim injected molding has lead to the molding of parts as described above with the attainment of several advantages. Namely, large quantities of two liquid, highly reactive starting products are rapidly delivered (in from about two to four seconds) and, at the same time, mixed and introduced into a mold where hardening to form the final molding takes place very quickly for polyurethane materials (from 1 to 2 minutes).
Before this new technology could be adopted for practical use, however, three problems had to be solved.
First, because of the high reactivity of the two starting components (polyisocyanate and polyisocyanate-reactive compounds), the reaction mixture has to be introduced into the mold in the shortest possible time. This time should at most be as long as the cream time. Thus, there was a need to develop high-performance axial and radial piston pumps which, when built into high pressure machines, would enable throughputs of from 2.5 to 6.5 kg/second to be obtained. Such machines are described, for example, in German Offenlegungsschriften Nos. 1,778,060 and 2,146,054 (British Pat. No. 1,382,741).
Secondly, the exact metering of the two components in a predetermined ratio, depending on the particular formulation, throughout the entire duration of the "shooting-in" phase is necessary. Additionally, thorough admixture thereof from the first to the last drop is essential to obtain a fault-free molding. Satisfactory admixture is made very difficult due to the high flow velocities of the two components and the extremely short residence time in the mixing chamber of the mixing head. This problem was solved by using so-called "static impingement" mixing heads operating on the "countercurrent injection principle" (cf. German Auslegeschrift No. 1,948,999 [U.S. Pat. No. 3,709,640] and also German Offenlegungsschriften Nos. 2,007,935 [U.S. Pat. No. 3,706,515]; 2,219,389 [U.S. Pat. No. 3,857,550] and 2,364,501 [U.S. Pat. No. 3,926,219]).
Thirdly, when the reaction mixture enters the closed mold, the air contained therein is almost instantaneously displaced. In order to prevent undesirable inclusions of air and, therefore, faults in the end product, the liquid flowing in has to push the air along in front of it in the form of a "flow front" and force it out at predetermined slot-like vents. Thus, in order to completely prevent turbulence during filling, the material has to enter the mold over a considerable width in the form of a laminar flow along the mold wall. This problem has been overcome through the development of a certain gating technique using so-called "film gates", of the type described in German Offenlegungsschriften Nos. 2,348,658 [U.S. Pat. No. 3,991,147] and 2,348,608 [U.S. Pat. No. 3,908,966]).
German Offenlegungsschrift No. 2,622,951 (British Pat. No. 1,534,258) describes how even highly reactive mixtures, i.e. one-shot mixtures, of active polyisocyanates, active aromatic polyamines, relatively high molecular weight polyhydroxyl compounds containing primary hydroxyl groups and strong catalysts, having cream times of less than one second, may be processed by this method. With such systems, the transition from the liquid to the solid phase is almost instantaneous, with the result that the liquid reaction mixture hardens on the walls of the mold.
It is possible by this process to fill voluminous and, at the same time, thin-walled (wall thickness &lt;3 mm), complicated mold cavities. Still-liquid material which continues to enter the mold under the filling pressure of the machine until the filling process is over would appear to force itself through between the peripheral zones of the molding hardening on the walls of the mold. This would appear to account for the fact that it is possible to produce moldings having greater weights than would appear theoretically possible by comparison of the cream time with the filling time (for a given filling volume per second). Thus, it is possible to exceed the filling time by up to 50% in relation to the cream time. On completion of the shot, the reaction mixture as a whole hardens so quickly that, in the case of highly reactive batches, the mold may be opened after less than five seconds and the molding removed therefrom.
Another advantage of these new systems is that their self-separating properties are better than those of the known systems i.e. for the manufacture of molded polyurethane foams so that it is possible to work without release agents at least if simple moldings such as plates are made.
Although it is possible in principle to fill voluminous mold cavities using the system according to German Offenlegungsschrift No. 2,622,951 (British Pat. No. 1,534,258), these known systems still do not fully satisfy practical requirements. Because of the extremely short cream and filling times, these known systems are still not optimally suitable for the production of very thin moldings. For example, the mass production of reversibly formable front and rear sections of automobiles are not practical because faults attributable to incomplete filling of the mold are often encountered. This applies in particular where the reaction injection molding machines currently available are used. Although it would be possible to compensate for the above-mentioned disadvantages of conventional systems by using specialized reaction injection molding machines having a considerably increased output, this would require a considerable additional capital investment in machinery.
An object of the instant invention is to improve the systems according to German Offenlegungsschrift No. 2,622,951 (British Pat. No. 1,534,258) in such a way that it is possible and practical to mass produce large-volume, thin-walled moldings weighing from 3 to 10 kg using available reaction injection molding machines. More particularly, this object is realized in the present invention by increasing the cream time or pourability of the conventional systems referred to above without, at the same time, significantly affecting the short in-mold time required.
In the present invention, this object is achieved by using polyhydroxy polyethers containing incorporated ethylene oxide units and secondary hydroxyl groups which are described in detail below. The achievement of this object in the present invention is surprising because, initially, it had been expected that the cream time would largely be determined by the more reactive amine component and not by the nature of the hydroxyl groups of the polyether. It had not been expected that only polyether polyols containing incorporated ethylene oxide units would produce the required effect. It was surprisingly found that corresponding polyether polyols having the same content of secondary hydroxyl groups, but without the incorporated ethylene oxide units, did not produce the required result. The systems according to German Offenlegungsschrift No. 2,622,951 which, by virtue of the mechanical properties thereof, are ideally suitable for the production of large-volume, flexible bodywork sections ("soft-face" elements) have, for example, a cream time of approximately two seconds and lead to moldings which may be mold-released after from 0.25 to 2 minutes. By using the polyether polyols of the present invention in otherwise the same formulation, it is possible to double the cream time for the same in-mold time.